My guess is that LucasArts decided not to use the NES any more because of the trouble they had with Maniac Mansion. According to this article, http://www.crockford.com/wrrrld/maniac.html they had to make a lot of changes to MM. Also, every nee console means a lot of work, so they have to choose carefully. The NES was five years old by 1998, so they probably decided to just focus on the newer consoles. That's my guess anyway.